The NRC conducted a special inspection into the undisclosed security violation at Vermont Yankee. The NRC’s report said it involved “an armed responder” — a security guard.

While the NRC said the security violation rose to a “Severity Level 4 violation,” the federal agency at the same time called it a “non-cited violation.”

Practically all of the information about the security violation has been kept secret from the public, following NRC security protocols, according to NRC spokesman Neil Sheehan.

According to the NRC announcement, “disclosure to unauthorized individuals could present a security vulnerability.”

The NRC’s Office of Investigations conducted the review, which was completed July 25. The first public information about the security lapse came in a letter dated Dec. 18 and released Friday by the NRC.

Yankee’s owner, Entergy Nuclear, identified the problem, “took immediate corrective actions to address any security-related vulnerability” and notified the NRC’s Region One office, according to a letter from Michael L. Scott, acting division director of the NRC’s Division of Reactor Projects.

Sheehan said the incident took place April 10 and the violation was committed by an armed security officer. No further details were released.

Robert Williams, spokesman for Entergy in Vermont, said the company was “absolutely committed to safe and secure operation of the plant.”

Williams said in an email the plant staff had alerted the NRC Region One office to the incident in April.

“Because Entergy took appropriate steps to address the situation and promptly report it to the NRC, the NRC concluded that it had minor security significance and today notified the plant that it was issued a Severity Level IV, non-cited violation,” said Williams, who declined to answer specific questions.

“At no time was the health, safety or security of the public or plant employees ever at risk,” the company statement concluded.

Sheehan said security violations occur at U.S. nuclear plants every year.

“There are usually multiple “green” (or low-level) violations at each plant during any given year,” Sheehan said Friday.

The April incident was not given a color grading, he said, because it was handled under “traditional enforcement.”

He said more serious findings are rare.

According to the NRC letter, Entergy Nuclear has 30 days to contest the findings of the inspection.